Intermittent hang on startup with r36030

Description

hrev36030 seems to have introduced some form of deadlock regression for me on startup, though I haven't yet managed to track down the exact issue. On some boots it succeeds, on others I don't get past the disks icon at boot. Getting into KDL reveals that both CPUs are running the idle thread at the time. Some details as follows, and serial attached:

Let me know what else might be useful, since this is at least fairly reliably reproducible here. Note that I'm also using the slab for kernel heap option if that makes any difference in this instance (since I notice the object cache is implicated in the backtraces above).